A portable electronic device is often equipped with a DC-DC converter used to convert the voltage generated by a battery to a power supply voltage used for an electronic circuit.
The voltage of the battery varies corresponding to its remaining capacity. For example, the voltage of a lithium ion battery may vary in the range of 3-4.2 V. Also, if an external power supply, such as an AC adapter, is used without using an incorporated battery, a voltage about 5 V may be input instead of the voltage of the battery. A DC-DC converter can generate a constant output voltage in spite of variation in the input voltage.
DC-DC converters are usually classified into three types—a step-down type, a step-up type, and a step-down/step-up type. The step-down type is used when the lower limit of the input voltage is higher than the target value of the output voltage. The step-up type is used when the upper limit of the input voltage is lower than the target value of the output voltage. The step-up/step-down type is used when the input voltage is higher or lower than the output voltage.
In order to reduce the power consumption of a battery in a portable electronic device, a standby mode is often adopted that can stop operation of at least part of the circuit when the device is not in use. Since the load of the DC-DC converter is very low in the standby mode, a PFM (pulse frequency modulation) method can be used to save more power than a PWM (pulse width modulation) method that performs switching constantly at a certain period. Japanese Kokai Patent Application Publication No. JP 11[1999]-235023 discloses a PFM type switching power supply device. That publication describes technology for reducing the ripple voltage when the power supply voltage is high, by making the power supply voltage dependent on the duty ratio of the PFM.
In a DC-DC converter, a voltage corresponding to the difference between the input voltage and the output voltage is usually applied to an inductor used for performing an energy converting operation. In the PFM method, the duration for applying the voltage to the inductor in one cycle of switching operation is fixed, and the output voltage is controlled by varying the repetition period of that fixing period. However, if the input voltage varies with the output voltage kept constant, the potential difference between input and output will vary, and the voltage applied to the inductor will vary. In the case of the PFM method, since the time of application of the voltage to the inductor is fixed, if the voltage changes, the variation rate of the current flowing through the inductor over time will vary, and the amplitude of the ripple in the output voltage will vary. For example, when the voltage difference between the input and output is increased, the voltage applied to the inductor is increased, and the variation rate of the current flowing through the inductor over time is increased. Therefore, when the application time of the voltage to the inductor is fixed, if the voltage difference between the input and output is increased, the ripple in the output voltage will increase. In other words, in a PFM-type DC-DC converter, the ripple in the output voltage varies significantly corresponding to the input voltage. This is a problem.
In recent years, the power supply voltage for electronic circuits has tended to become lower and lower. Since ripple in the power supply voltage has a significant influence on the reliability of the device, stricter control of the ripple voltage is desired.
An objective of the present invention is to address the aforementioned problems by providing a switching power supply device and a control device therefor which can control the variation of ripple in the output voltage corresponding to the variation in input voltage.